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Commuters who ask Siri to dial a spouse or friend on the way home from work have a new reason to rethink that ritual.

Distraction lingers even after drivers disconnect from hands-free systems, according to University of Utah researcher David Strayer. The voice-activated systems affect drivers' attention for up to 27 seconds — the length of three football fields for a car traveling at 25 mph.

"Just because it's in the car doesn't mean that it's a good idea to use it," said Strayer, adding that the recent "explosion of technology" allows people to "do things that you really shouldn't do while driving."

Strayer's warning pertains to in-car information systems, as well as smartphone assistants: Microsoft Cortana, Apple's Siri and Google Now. Google Now was slightly less distracting than the others, but no system was found to be less than moderately distracting.

Dialing a number, calling someone, changing music and sending texts all are culprits that can rob drivers' attention, even those who are accustomed to the systems, according to the research released by the U. and the AAA Foundation for Traffic Safety on Thursday.

Among the car systems tested, all in 2015 models, six received a distracting rating of high, and three were moderate. One — the Mazda 6 — was deemed very highly distracting.

No version is seamless, so Strayer and co-author Joel Cooper say it's best to limit multitasking as much as possible — especially as drivers get older.

Among older drivers in the group of participants ranging from 21 to 70, Cooper said, using the in-car systems "was even harder and more frustrating and more demanding."

The data has strong implications for public safety, Strayer said.

Nationally, roughly 3,100 people died and 424,000 others were injured in crashes involving driver distraction, according to the U.S. Department of Transportation.

In Utah, cellphones are the lead cause of distracted driving deaths, show health department statistics from 2012. Cellphones accounted for roughly 14 percent of the 4,806 distracted driving crashes in the same year, according to the department.

When driving at 25 mph, users of moderately distracting phone and car systems experienced 15 seconds of distraction; those using the most distracting system lost focus for 27 seconds.

The results indicate that motorists could fail to notice stop signs, pedestrians and other vehicles, said Peter Kissinger, president and CEO of the AAA Foundation for Traffic Safety, in a statement.

The subject is not new for Strayer and his team, who in 2006 published research suggesting that anyone who gabs on the phone while driving — even using a headset or other hands-free device — is as impaired as a drunken driver.

Strayer said Wednesday his team has received $2.5 million for related research since 2013. Two more phases are planned.

To conduct the study, researchers recruited drivers to cruise at no more than 25 mph around a 2.7-mile route through Salt Lake City's Avenues neighborhood. The drivers used their cars and phones to dial numbers, call contacts and fiddle with the radio.

To measure the effects of the technology, they wore wearable LED lights that flashed red every three to five seconds at the edge of a driver's left eye. Drivers pressed a switch when they saw the light — and video cameras recorded them. The participants also were surveyed about their perceived level of distraction.

About 260 people used the car systems, and 65 used the personal assistant. No one with an at-fault accident in the past five years was allowed to participate.

Some participants took the cars home for five days to get familiar with the systems, then returned for reassessment of the mental workload of using them.

The U. team in recent weeks has presented the research to car makers and federal regulators. The next phase of research, due out next year, focuses on the effect of touchscreens on drivers.

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